1. Field of the Invention
The present invention relates to a low reflection member excellent in preventing the reflection of external light for use in image display devices such as a liquid crystal display (LCD), plasma display (PDP), cathode ray tube (CRT), and Electroluminescence device (EL).
This application is based on patent applications filed in Japan (Japanese Patent Applications Nos. Hei 11-310343 and 2000-303995), whose contents are incorporated herein by reference.
2. Description of the Related Art
Various image displays are provided with a low reflection member in order to prevent the reflection of external light.
It has been known that formation on an outermost layer of a transparent substrate made of a film or glass of a low refractive index layer made of a material having a lower refractive index than that of the substrate to an optical thickness of xc2xc of the visible light wavelength will decrease the reflectance resulting composite due to interference.
The structure of low reflection member includes one in which only a low refractive index layer in a single layer is provided on at least one side of the substrate and another in which a high refractive index layer and a low refractive index layer are laminated in order on at least one side of the substrate to make a multi-layer structure. The refractive index and film thickness of each layer for exhibiting a good anti-reflection function can be calculated using a known equation. According to Yoshida, S. and Yajima, H., xe2x80x9cThin films/Optical devicesxe2x80x9d, Tokyo University Publishing, the conditions under which the incident light that enters a low refractive index layer perpendicularly is not reflected and transmits at an intensity of 100% must satisfy the following equations.
n12=nsxe2x80x83xe2x80x83(1)
n1d1=xcex0/4xe2x80x83xe2x80x83(2)
wherein n1 indicates the refractive index of the low refractive index layer, ns indicates the refractive index of the substrate or the high refractive index layer, d1 indicates the thickness of the low refractive index layer, and xcex0 indicates the wavelength of light.
In order to fully (100%) prevent the reflection of light, the refractive index of the low refractive index layer in the equation (1) must be the square root of the refractive index of an underlying layer (substrate or high refractive index layer) and the film thickness of the low refractive index layer must be identical with a value calculated according to the equation (2) above from the refractive index of low refractive layer and the wavelength of light selected in the equation (1) above.
In the equation (1), the refractive index, ns, of a typically used substrate or high refractive index layer is in the range of 1.45 to 2.10, and hence the refractive index, n1, of low refractive index layer that is fit therefor will be in the range of 1.20 to 1.45. In the case, where the reflectance in the visible light region is to be made lowest in the equation (2), the film thickness, d1, will be suitably about 100 nm.
Most of anti-reflection coatings are formed by film formation methods under a vacuum, such as a vacuum vapor deposition method and a sputtering method. When plastic films are selected as a substrate, the substrate has a low thermal deformation temperature so that it cannot be heated sufficiently and the resulting anti-reflection coating tends to have insufficient adhesion to the substrate. Members to which these methods can be applied are limited to those having relatively small sizes and they are disadvantageous in that they are not only unsuitable for continuous production but also require high production costs. Hitherto, spectacle lenses have been provided with anti-reflection coatings by such a vacuum film formation method.
On the other hand, recently, image displays, typically LCD, PDP, CRT and EL, (hereafter, referred to simply as xe2x80x9cdisplaysxe2x80x9d) are being frequently used in various fields including televisions and computers, and are being rapidly developed. The development of such displays is focused on higher precision images, higher quality images, and lower prices, and as a natural consequence, there has been an increasing demand for anti-reflection. Accordingly, attention has been paid to development of low reflection members produced by solvent coating, which enables production of large area coating and continuous production as well as production at low costs.
Naturally, the characteristics such low reflection members require include one that they have a refractive index of 1.45 or less. Besides, in order for them to be useful under the condition of use at the outermost surface of a display, it is required that they have high adhesion and high resistance to scratching. However, most of conventional low reflection members have been designed with attention being paid to only lower refractive index and are not always satisfactory with respect to their resistance to scratching.
The means for obtaining a low refractive index layer by solvent coating are roughly classified into two methods. One is a method which realizes a lower refractive index by use of a material containing fluorine and the other is a method which deposits fine particles, etc. on the surface of a film to provide pores and introduces air therethrough to achieve a lower refractive index.
Classified by the material that constitutes the low refractive index layer, they are roughly divided into three techniques, that is, a technique which cures hydrolyzable silane compounds by sol-gel reaction (JP-A-9-24575), a technique which uses fluorine-containing organic materials (JP-A-2-19801), and a technique which uses fine particles with a low refractive index. Also, as combinations of the above three, there have been proposed a technique which combines a fluorine-containing organic material with a hydrolyzable silane compound (JP-A-7-331115), a technique which combines a fluorine-containing organic material with fine powders with a low refractive index (JP-A-6-230201), a technique which combines a hydrolyzable silane compound with fine particles with a low refractive index (JP-A-8-211202), etc.
However, none of the techniques is furnished with practically sufficient anti-reflection characteristic along with anti-scratching and productivity where a plastic film is selected as a substrate.
For example, the method that uses fine particles with a low refractive index produces an increased refractive index as the ratio of the binder increases, so that a sufficient anti-reflection property cannot be obtained. On the other hand, a lesser amount of the binder results in a decrease in film strength so that the obtained low reflection member cannot be used.
In the method that uses a fluorine-containing organic material, a radical polymerization product of a fluorine-containing acrylic compound is mainly used. The radical polymerization-type compounds undergo inhibition of their polymerization by the oxygen in the air so that no sufficiently cured films can be obtained, with the result that the obtained films are deficient in hardness and their volume shrinkage at the time of curing is great so that problems of adhesion to the substrates and peeling occur therefrom.
The method that uses hydrolyzable silane compounds has problems in that it requires high temperature or a long curing time. This causes such problems as the deterioration of the substrate and that the drying step requiring a long time, which incurs high production costs.
The present invention has been made in order to solve the above problems and it is an object of the present invention to provide a low reflection member comprising a low refractive index layer that has a low refractive index sufficient for exhibiting an anti-reflection effect, is fabricated at relatively low temperatures by solvent coating using an common solvent, and has high adhesion and anti-scratching properties.
The low reflection member of the present invention comprises a transparent substrate and a low refractive index layer having a refractive index lower than that of the transparent substrate, wherein the low refractive index layer comprises a cured product of a mixture of a hydrolyzable silane compound and/or a hydrolyzed product thereof and a cure promoting component composed of at least one selected from phosphoric acids, sulfonic acids and silicon dioxide flux, the hydrolyzable silane compound being at least one selected from a silane compound represented by the following general formula [1], a silane compound represented by the general formula [2], a compound or polymer represented by the general formula [3]
R1axe2x80x94SiX4xe2x88x92a 0xe2x89xa6axe2x89xa62xe2x80x83xe2x80x83[1]
X3Sixe2x80x94R2xe2x80x94SiX3xe2x80x83xe2x80x83[2]
Yxe2x80x94(Si(OR3)2O)nxe2x80x94Yxe2x80x83xe2x80x83[3]
(wherein X represents any one of Cl, Br, NCO and OR4, Y represents H or an organic group having 1-20 carbon atoms, R1, R2, R3 and R4 represent each an organic group having 1-20 carbon atoms, and n is an integer of 1-30).
The low reflection member of the present invention allows curing to occur at low temperatures as low as about 40xc2x0 C. and it can be coated on a film. Furthermore, the low reflection member of the present invention has high film strength that has never been attained by the conventional cured products of silanes as well as excellent adhesion and anti-scratching properties.
Here, preferably, the low refractive index layer is a cured product of a mixture containing 1-30 parts by weight of the cure promoting component per 100 parts by weight of the hydrolyzable silane compound and/or hydrolyzed product thereof.
Furthermore, the hydrolyzable silane compound is a mixture of a silane compound having 4 hydrolyzable groups and one or both of a silane compound represented by the general formula [4] and a silane compound represented by the general formula [5] below, thereby improving refractive index and film strength in a good balance.
R5axe2x80x94SiX4xe2x88x92a 1xe2x89xa6axe2x89xa62xe2x80x83xe2x80x83[4]
X3Sixe2x80x94R6xe2x80x94SiX3xe2x80x83xe2x80x83[5]
(wherein X represents any one of Cl, Br, NCO and OR4, R5 and R6 represent each an organic group having at least one fluorine atom, and R4 is as defined above).
Furthermore, a high refractive index layer may be provided between the low refractive index layer and the transparent substrate.
Polarizing plates may be used as the transparent substrate.